Various heat transfer recording methods have been known so far. Among these methods, dye diffusion transfer recording systems attract attention as a process that can produce a color hard copy having image qualities closest to that of silver salt photography (see, for example, “Joho Kiroku (Hard Copy) to Sono Zairyo no Shintenkai (Information Recording (Hard Copy) and New Development of Recording Materials)” published by Toray Research Center Inc., 1993, pp. 241-285; and “Printer Zairyo no Kaihatsu (Development of Printer Materials)” published by CMC Publishing Co., Ltd., 1995, p. 180). Moreover, this system has advantages over silver salt photography: it is a dry system, it enables direct visualization from digital data, it makes reproduction simple, and the like.
In this dye diffusion transfer recording system, a heat-sensitive transfer sheet (hereinafter also referred to as an ink sheet) containing dyes is superposed on a heat-sensitive transfer image-receiving sheet (hereinafter also referred to as an image-receiving sheet), and then the ink sheet is heated by a thermal head whose exothermic action is controlled by electric signals, in order to transfer the dyes contained in the ink sheet to the image-receiving sheet, thereby recording an image information. Three colors: cyan, magenta, and yellow, are used for recording a color image by overlapping one color to other, thereby enabling transferring and recording a color image having continuous gradation for color densities.
General paper may be used as a support of an image-receiving sheet in this dye diffusion transfer recording system, and it enables the image-receiving sheet to be produced at low costs. In an image-receiving sheet using such paper as the support, a layer having high cushion properties, for example, a foam layer made of a resin and a foaming agent, is positioned between the support and a receptor layer, to provide cushion properties in order to supplement cushion properties of the support, thereby improving the adhesion between an image-receiving sheet and an ink sheet. Also, an intermediate layer is further laid between this foam layer and the receptor layer, to prevent the foam layer from being broken (flatten) by heating during printing. However, there are caused certain problems in current image-receiving sheets because of the fact that this intermediate layer is being formed by using an organic-solvent-type resin coating solution. The problems are that this coating solution breaks down air cells and voids in the foam layer, and thus, desired cushion properties are not attained, resulting in voids and density unevenness in the formation of an image, and further reduction in the heat insulation property of the foam layer is caused, resulting in diffusion of the calories required to transfer dyes, in the direction of the backside of the image-receiving sheet, bringing about reduction in sensitivity that is required for printing.
For example, JP-A-8-25813 (“JP-A” means unexamined published Japanese patent application) discloses use of an aqueous-type coating solution to form an intermediate layer between a foam layer and a receptor layer, to utilize subtle unevenness of the foam layer as it is, as the surface form of the receptor layer. However, in this method, the receptor layer is applied after application of the foam layer on a support and drying of the foam layer under heating, and therefore, there is the problem that not only do many image defects arise due to the delicate unevenness formed on the receptor-layer surface, but also the receptor layer has insufficient sensitivity and is expensive. Also, JP-A-11-321128 discloses that an intermediate layer containing, as its major components, hollow particles and a polymer resistant to an organic solvent, is formed between a support and a receptor layer; and also, JP-A-5-147364 discloses that a resin layer including a dye receptor layer is made to contain a hollow capsule. In these methods, however, the receptor layer is likewise applied after the intermediate layer and the resin layer are applied and dried under heating, and therefore, there is the problem that not only do many image defects arise due to the unevenness formed on the receptor-layer surface, but also the receptor layer has insufficient sensitivity and is expensive.